Obama vows to protect South Korea

South Korea has said it will join a US-led initiative to intercept ships suspected of spreading weapons of mass destruction.

It comes as the United Nations swiftly condemned North Korea for testing a powerful nuclear bomb.

And in a further sign of the North's mounting stand-off with the world, a report said the country was probably preparing to fire short-range missiles on Tuesday off its western coast.

The UN Security Council said the test was a "clear violation" of a 2006 resolution banning North Korea from conducting nuclear development, and that it would start work immediately on a new resolution that could result in even stronger measures.

Russian officials said the nuclear bomb that the North detonated underground on Monday was comparable to those that obliterated Hiroshima and Nagasaki, raising fears that the communist country could spread such technology abroad.

South Korea's Yonhap news agency said North Korea had banned ships from waters off its western coast and would probably fire short-range missiles as early as Tuesday.

US president Barack Obama told South Korean president Lee Myung-bak that the United States would protect his country from any possible North Korean aggression and called for a "strong resolution" by the UN, Mr Lee's spokesman Lee Dong-kwan said after the two leaders spoke by telephone.

South Korea, which previously stayed out of the US-led Proliferation Security Initiative in order to pursue reconciliation efforts with North Korea, set aside its reservations and announced it would join the pact immediately.

The programme involves stopping and searching ships suspected of carrying nuclear, chemical and biological weapons, materials to make them, or missiles to deliver them. North Korea has previously warned the South that joining the programme would be considered an act of war.

Earlier, Mr Obama had criticised Pyongyang's "blatant defiance" of existing resolutions and British prime minister Gordon Brown condemned the test as a "danger to the world". Russia's foreign ministry called it "a serious blow to international efforts" to prevent the spread of nuclear weapons.